Recent years have seen rising demand for lithium ion batteries, nickel-hydrogen batteries, and other such secondary batteries as power sources installed in vehicles, as well as power supplies for personal computers and portable terminals. In particular, it is anticipated that lithium ion batteries can be used to advantage as high-output power supplies installed in vehicles because of their light weight and high energy density.
Among the lithium ion batteries of this type, a cylindrical battery, for example, has a positive electrode sheet and a negative electrode sheet coiled with a separator sheet in between, and the resulting coiled electrode body is housed in a metal battery case. A positive electrode current collecting plate and a negative electrode current collecting plate are attached respectively to the two ends of the coiled electrode body in the coiling axis direction (that is, one end is the positive electrode, and the other end is the negative electrode). Typically, the positive electrode current collecting plate is welded to the lid (positive electrode terminal) of the battery case, while the negative electrode current collecting plate is fixed by welding (such as resistance welding) to the bottom face of the battery case. With a battery of this form, since current can be taken out from the electrode body via the positive and negative current collecting plates, there is a reduction in collecting resistance, and charging and discharging efficiency can be improved. Patent Document 1 is an example of prior art related to this type of current collecting plate (and particularly a current collecting plate that is welded to the bottom face of a battery case).
Patent Document 1: Japanese Patent Application Laid-Open No. 2006-100214
With a battery configured as above, however, a problem is that welding spatter (such as metal particles that melt during welding and scatter in the form of sparks) and so forth and other such foreign matter tends to be produced in the welding of the current collecting plate to the bottom face of the battery case. When welding spatter or the like occurs, not only does it scatter around the periphery of the weld and adversely affect the welding quality, but it can work its way into the battery case through the gap between the current collecting plate and the battery case, where it can become a source of diminished battery performance, such as internal shorting of the electrode body. This type of welding spatter is difficult to detect when it occurs, and consequently it is currently impossible to prevent the scattering of this spatter merely by adjusting the welding conditions.